1. Field of the Invention
This invention relates to a surface-condition inspection method and apparatus for inspecting conditions of a surface to be inspected, and more particularly, to a surface-condition inspection method and apparatus for discriminating the presence of defects or foreign particles on a reticle (photomask) used when manufacturing devices, such as semiconductor memories, liquid-crystal displays, magnetic heads or the like, or on a pellicle for protecting the reticle from foreign particles.
2. Description of the Related Art
In general, in the IC (integrated circuit) production process, circuit patterns formed on a substrate, such as a reticle, a photomask or the like, are transferred onto the surface of a wafer coated with a resist using a printing apparatus (e.g., a stepper or a mask aligner).
If foreign matter, such as pattern defects, dust particles or the like, is present on the surface of the substrate at the time of circuit pattern transfer, the foreign matter is also transferred, causing a decrease in the yield of the IC production.
In particular, when circuit patterns are printed onto the surface of a wafer by a step-and-repeat method using a reticle, if even one harmful foreign matter particle is present on the surface of the reticle and is printed onto the entire surface of the wafer, a great decrease in the yield of the IC production is caused.
Accordingly, the ability to detect the presence of foreign matter on a substrate in the IC production process is indispensable, and various kinds of inspection methods have been proposed for that purpose. In general, a method which utilizes the property that foreign matter isotropically scatters light has been used.
FIG. 1 is a diagram showing a schematic configuration of a principal part of a conventional surface-condition inspection apparatus.
In FIG. 1, light beam 1a emitted from laser 1 is incident upon beam expander 3 via pinhole plate 2. Expander 3 transforms the incident light beam 1a into a parallel light beam while increasing the beam's diameter, and the resultant light beam is incident upon polygonal mirror 4. After being reflected by polygonal mirror 4, light beam 1a is condensed onto reticle 6 by scanning lens 5. By rotating polygonal mirror 4, light beam 1a scans the surface of reticle 6 in a direction orthogonal to the plane of FIG. 1. Reticle 6 moves in the direction of two-headed arrow S.sub.1 in synchronization with the scanning by the polygonal mirror 4. The entire surface of reticle 6 is thereby subjected to optical scanning.
If light beam 1a hits foreign matter, such as a pattern defect, a dust particle or the like, on the surface of reticle 6, scattered light is generated from the foreign matter. Light-receiving lens 7 of detection system BD condenses back-scattered light from the foreign matter.
The object field of light-receiving lens 7 includes the beam scanning line on reticle 6. The amount of light received by light-receiving lens 7 is limited by aperture stop 8 which defines the entrance pupil of the lens 7. Scattered light passing through aperture stop 8 is condensed onto the surface of field stop 10, and imaging lens 9 images reticle 6 on the surface of field stop 10. Field stop 10 has a slit-like opening, and has a function of blocking flare light and the like other than scattered light from the foreign matter, passing only scattered light from the beam scanning line, and guiding the light to light-receiving unit 12 via condenser lens 11. The scattered light from the foreign matter is detected by light receiving unit 12.
FIG. 2 is a diagram illustrating the directional property of circuit patterns on the surface of a reticle in an actual production process. Typical conventional circuit patterns mainly comprise pattern A, pattern B and pattern C whose directions with respect to the x axis are 90 degrees, 0 degree and 45 degrees, respectively. The apparatus shown in FIG. 1 receives a greater amount of diffracted light from pattern A than from the other patterns.
FIG. 3 shows a portion of the optical path of the apparatus of FIG. 1 when pattern A is formed on the surface of the reticle 6. In FIG. 3, to ease understanding of the paraxial relationship between the light-projecting system and the light-receiving system, the light-projecting system and the light-receiving system are illustrated on a straight line.
FIG. 4 illustrates a state in which pattern-diffracted light S.sub.PO, though weak, appears on the surface of aperture stop 8 of detection system BD as a one-dimensional point sequence. The diameter .phi..sub.s of one spot of the pattern-diffracted light in the direction of the incident cross section (incident plane) depends on the line width and the pitch of a repeated pattern within the diameter of the inspection beam 19 when the beam 19 stands still, the wavelength of the inspection beam 19, and the focal length fd of light-receiving lens 7. On the other hand, the diameter .phi..sub.m of the one spot of the pattern-diffracted light in the scanning direction of the inspection beam 19 is determined by the product of the opening angle of the incident beam (i.e., the numerical aperture NAj of the light projecting system) and the focal length fd of light receiving lens 7.
Accordingly, when pattern A is present at central portion P.sub.O of the scanning line on the reticle 6, the pattern-diffracted light S.sub.PO can be blocked by providing light-blocking plate 13 as shown in FIG. 5 in front of aperture stop 8.
However, the following problems arise when the aperture of aperture stop 8 is partly limited by light-blocking plate 13.
That is, when inspection is performed in a state in which the reticle 6 has rotated by an error amount within the horizontal plane because of a mechanical positioning error, pattern-diffracted light S.sub.PO laterally deviates on the surface of aperture stop 8 by an amount determined by the product of the error and the focal length fd. As a result, the pattern-diffracted light passes through light-blocking plate 13, and is erroneously detected by light-receiving unit 12.
When a circuit pattern is disposed at a portion around the beam scanning line (portion P.sub.L in FIG. 3), pattern-diffracted light S.sub.PL on aperture stop 8 laterally deviates from the central portion. In general, this occurs when accuracy in the alignment of the pupils of the light projecting system and the light-receiving system is insufficient. In primary approximation, such lateral deviation does not occur if the reflecting point P.sub.R of polygonal mirror 4 and the central point P.sub.f of aperture stop 8 are in a conjugate relationship, and pattern-diffracted light S.sub.PL coincides with pattern-diffracted light S.sub.PO on the stop. This corresponds to a case in which the pupils are in alignment.
Actually, however, an adjustment for maintaining such a relationship requires accuracy, and a residual component usually remains. For example, as is apparent from FIG. 3, if point P.sub.f ' is conjugate to point P.sub.R, pattern-diffracted light S.sub.PL laterally deviates on the surface of aperture stop 8, and passes through light-blocking plate 13.
Even though point P.sub.f ' is present at an ideal paraxial position, the diffracted light in some cases passes through light-blocking plate 13 due to the aberration of light-receiving lens 7. Strictly speaking, the reflecting point itself of the polygonal mirror laterally deviates in accordance with the rotation of the mirror. This lateral deviation corresponds to deviation on light-blocking plate 3. This is one of the factors which causes difficulty in adjustment.
In accordance with an increase in the degree of integration of semiconductor chips, circuit patterns having directions other than 0 degree, 90 degrees and 45 degrees have been used. For example, a circuit pattern D shown in FIG. 2 slightly rotates by an angle .theta. with respect to circuit pattern A. As shown in FIG. 17, diffracted light S.sub.PO generated by such a circuit pattern appears accompanied by lateral deviation and rotation on aperture stop 8.
If the size of light-blocking plate 13 is increased in order to overcome the above-described problems, the amount of blocking of scattered light from foreign matter also increases, causing a decrease in the S/N ratio.